A problem which occurs in data communication is corruption due to, for example, channel noise, resulting in the loss of part or all of the message data. Two transmission protocols for use with mobile data systems are disclosed in a paper "Predicting the range and throughput of mobile data systems" by P. J. Mabey reported in the IEEE Vehicular Technology Conference, May 1982, San Diego, USA, pages 370 to 374. Before discussing these protocols reference will be made to FIG. 1 of the accompanying drawings which shows a typical data format. The format begins with a preamble PR of 16 bit-reversals 1010 . . . 10 to enable bit synchronization in a data demodulator. The preamble is followed by a 16-bit synchronization sequence SYN to enable code word framing. This is followed by the information which is transmitted in 64-bit code words based on a cyclic error detecting and/or correcting code, each code word containing 48 information bits and 16 error-check bits. The first code word ACW of the message contains addressing information and some data, and is sufficient for short message applications such as status reporting and precoded messages. For longer data messages additional code words DCW1 to DCWn are concatenated onto the first code word ACW as required to accommodate data.
The first retransmission protocol described in the above mentioned paper is termed a simple retransmission protocol. Essentially in this protocol the sending equipment transmits the entire message and then waits for an acknowledgement from the receiving equipment. If no acknowledgement is received or if a retransmission is requested, the sending equipment retransmits the entire message and again waits for an acknowledgement. The message may be retransmitted up to a predetermined maximum number of times, after which if no acknowledgement has been received, the transaction is terminated. For each transmission the receiving equipment must successfully decode the synchronization word SYN and address code word ACW before it can decode the data code words DCW. Decodable data words may be stored and undecodable data words are discarded. The receiving equipment uses data words from the retransmissions, if necessary, to complete the message and the receiving equipment transmits a transaction completion acknowledgement only when it has assembled the whole message (or receives a repeat of a message it has previously completed, because its previous acknowledgement was not decoded or was delayed). Because each retransmission comprises the entire message this simple retransmission protocol can lead to a low throughput. Furthermore it is not suitable for group calls where the receiving equipment does not send acknowledgement signals.
The second of the known retransmission protocols is termed selective retransmission protocol. It enables a higher throughput by avoiding retransmission of data code words already decoded successfully. In this second protocol the receiving equipment transmits an acknowledgement to every transmission that it receives to specify which data code words must be retransmitted, thus improving throughput. In the event of an acknowledgement not being received successfully, the sending equipment repeats its previous transmission. The transaction is complete when an acknowledgement is received which indicates that the receiving equipment has assembled the whole message, or when the sending equipment has made the predetermined maximum number of transmissions. This second protocol also relies on acknowledgements being sent by the receiving equipment which makes it unsuitable for group calls.
Additional problems which arise with retransmission protocols are "duplication" of messages at the receiving equipment when retransmissions are made, the "omission" of messages at the receiving equipment due to corruption in transmission, and "lost" messages due to a sender interpreting an acknowledgement for the wrong message because, for example, the acknowledgement was late. These problems are overcome in line protocols, known per se, by numbering schemes. However, line protocols have the disadvantage of signalling overheads for call establishment or call initalization. Also they are not optimised for group calls.
It is known that efficiency can be improved by transmitting data as a series of segments but, as far as is known, such protocols require a call establishment phase which is an undesirable overhead, particularly for small amounts of data.
An object of the present invention is to overcome these disadvantages when communicating data and provide a more efficient and flexible system.